Magnetic device



S. G. BEST MAGNETIC DEVICE Dec. 25, 1951 2 SHEETS-SHEET 1 Fixed Oct. 28,1947 NTOR I e3 6113952 ATTORNEY Dec. 25, 195] s BEST 2,579,723

MAGNETIC DEVICE Filed 00%.. 28, 1947 2 SHEETSSHEET 2 Patented Dec. ,25,1951 2,519,723 MAGNETIC DEVICE Stanley G. Best, Manchester, Conn.,assignor to United Aircraft Corporation, East Hartford, Coma, acorporation of Delaware Application October 28, 1947, Serial No. 782.535

17 Claims. 1

This invention relates to a magnetic device and particularly to amagnetic device having an armature whose movements are substantiallyproportional to the voltage impressed on, or the amperage flowingthrough, the electromagnetic coils or solenoid.

An object of this invention is a magnetic device having a linearlymovable armature which is moved, as a result of an electric currentsignal impressed on the electromagnetic coils or sole noid, a distancesubstantially proportional to the strength of the signal impressed onthe coils.

Another object of this invention is a magnetic device combined with acontrol valve for moving said valve in accordance with, the strength ofa signal impressed on the device.

Other objects and advantages will be apparent from the specification andclaims, and from the accompanying drawings which illustrate what is nowconsidered to be a preferred embodiment of the invention.

Fig. 1 is a vertical cross-section taken along the lines l-l of Fig. 4of the proportional magnetic device connected to a control valve.

Fig. 2 is a section taken along the line 2--2 of Fig. 1. i

Fig. 9 is a section taken along the line 1-3 of Fig. 1.

Fig. 4 is a section taken along the line 4-40! Fig. l.

Fig. 5 is a force-displacement diagram for constant' current in aconventional elec'tromagnet.

Fig. 6 is a displacement-current curve for a conventional .electromagnetshowing the nonlinear relation between the current and displacement.

Fig. 7 is a diagram showing the flux paths in applicant's proportionalmagnetic device.

Fig. 8 is a force displacement diagram for constant current in theproportional magnetic device.

Fig. 9 is a theoretical displacement-current curve for the proportionalmagnetic device.

Fig. 10 is a displacement-current curve showing'the type of controlwhich has been obtained in practice.

.pymagnetic device which will have a displacement substantiallyproportional to the applied idl tage or current is desirable for manytypes di -controls, and finds particular utility in a propeller pitchcontrol in which the input voltage or current may be proportional to aspeed variation from a predetermined standard or reference speed. As thespeed variations may be above or below, or both above and below, thestandard,

such a solenoid should be double-acting to correct all such speedvariations.

In the magnetic device shown in Fig. 1, a linearly movable armature I2is centered between end plates 22 and 24 by means of opposed springs l8and 20 having a linear spring rate. A cylindrical casing or yoke 26surrounds and is concentric with the armature l2 and springs l8 and 20and encloses a pair or concentrically arranged, annular, axially spacedelectromagnets or coils Ill and I4 located concentric with and betweenthe armature I2 and the shell 26 with their magnetic axes parallel tothe armature axis. Midway between end plates 22 and 24, is a permanentlymagnetized washer 28 forming an. annular permanent magnet with itsmagnetic axes extending radial or normal to the armature axis andlocated between and concentric with electro magnetic coils or solenoidsI0, l4 and snugly fitting within a depending flange 21 of the easing 26and closely surrounding armature l2. Armature i2 is guided linearlyalong the central axis of the magnetic device by means of corrugateddiscs 30 and 32 which serve to support armature 12 in its axial positionat all times. Disc 30 is held in position by a cap 34 and disc 32 isheld in position by a washer 36. The discs are slotted as shown in Fig.4 and a vent passage 31 connects the areas under the discs. The aboveassembled magnetic device is mounted on a casing 38 which is shown asthe governor casing now usually provided on the nose oi airplane enginesand enclosing. well known propeller pitch governor mechanism. Such agovernor casing including the governor mechanism is shown in WoodwardPatent 2,204,640. This patent also shows schematically how the governormechanism is driven by the engine and hydraulically connected to thepropeller. The governor shown in Fig. 1 of this application, whilesimilar to the governor of Woodward Patent 2,204,640 discloses adouble-acting valve 40 having a valve opening linearly proportional tothe valve movement from the centered position shown, in place of thesingle-acting valve shown in the Woodward patent. Armature l2 andlinearly movable valve member 40 are arranged in axial alignment anddirectly secured together so that each movement of the armature resultsin a corresponding and equal movement of valve member 40 and. a changein the valve opening linearly proportional to, the valve movement. Inthe device shown in Fig. l the sleeve 42 is driven by the engine in amanner similar to that shown in the Woodward patent, but the fiy-weightsoi the Woodward patent are omitted and their functions are replaced bythe above described magnetic device. The gear 44 is a part of the gearpump supplying oil under pressure in the well known manner to pressureline 46 from which it may be distributed by valve 40 to either line 48or line 50. When one of lines 48 or 50 is connected with the pressureline 46, the otherline is connected by valve 40 with a drain 52. Lines48 and 50 connect to opposite sides of a double-acting propeller pitchchanging motor such as shown in Patent No. 2,402,065. Continuousrotation of sleeve 42 not only drives the gear 44, but provides rotatingor sliding friction at all times between sleeve 42 and valve 40, thusreducing the friction between these two elements and rendering thegovernor valve more sensitive to impulses of the magnetic device.

A conventional double-acting 'electromagnet has a very non-lineardisplacement vs. current characteristics. Such a magnet might be similarto the one shown in Fig. 1, except that the permanent magnet shown inthe center of the solenoid in Fig. 1 would be replaced by anunmagnetized shell portion of good magnetic flux conducting material. Insuch a conventional electromagnet, when coil II] is energized thearmature I2 would move upwardly as seen in Fig. 1, regardless of thepolarity of the applied voltage. When coil [4 is energized, the armature12 would move downwardly regardless of the polarity of the appliedvoltage. Hence if this conventional electromagnet were utilized tocontrol a valve such as a propeller governor valve, the speed responsivemechanism would have to apply voltage to one coil or the other coildepending upon the direction of motion desired. When coil It isenergized, the magnetic flux travels across gap I5 emerging from thearmature near its midpoint to complete the circle around coil Ill. Theflux in traveling across gap it would pull the armature upward. For agiven gap the force is proportional to the square of the current. As thearmature moves upward and diminshes the gap, the fiux, and the forceproduced by this flux, increases. The relationship between the current,the force produced by this current, and the displacement of the armaturemay be shown in a curve. Typical force displacement characteristics ofthe conventional double-acting electromagnet for constant current areshown in solid lines in Fig. 5. From this diagram it will be noted thatwhen coil I is energized by a current of one unit, a force of about onequarter unit is exerted which increases as the displacement increases.As a current of three units is impressed, the force varies in accordancewith a curved line from about 2.25 units to about 4.5 units. Opposedsprings l8 and 20 acting to center the armature I2 are linear springs,that is they have a substantially linear spring rate so that theircharacteristics can be represented by a straight line showing that theforce exerted by the spring increases substantially proportional to thedisplacement. If the curve, representing the spring characteristics, issuperimposed on the force-displacement curves of the electromagnet asshown by the dotted line in Fig. 5, the points of intersection willindicate the points of equilibrium between the forces exerted by theelectromagnet and the forces exerted by the springs. It will be notedfrom Fig. that these equilibriums are very unequally spaced for equalincrements of current. This feature may be shown 4 shows that equalincrements of current produce very unequal increments of displacement.

A non-linear spring such as a Belleville washer, which stiiIens up as itis displaced, might theoretically be used to obtain a lineardisplacement-current curve. This, however, would be impractical for tworeasons. First, it would be diflicult to align the Belleville spring sothat its non-linearity matches that of the electromagnet and secondly,at the same time that the nonlinear functions are aligned, toadditionally align the control valve in its center position. The springshould be adjusted to center the pilot valve at zero current in theelectromagnet.

Ithas been found possible, by utilizing a polarized magnetic deviceshown'in Fig. 1, to provide a magnetic device having linear orsubstantially linear characteristics. In such a polarized magneticdevice, current applied to either coil l0 or l4 or both coilssimultaneously will displace the armature 12. The direction ofdisplacement depends, not on which coil is energized, but on thepolarity of the energized coil.

The reason for this is that the permanent magnet 28 midway between theends of the magnetic device has low permeability for the electromagneticflux induced by the energized coils l8, l4. The permanent magnet,therefore, acts like a large air gap with respect to the electromagneticflux so that the electromagnetic flux travels the entire length ofcasing 26 and across the end plates 22 and 24 and through the entirelength of the armature l2. Thus the electromagnetic flux passes throughboth gap l6 and gap 54. The path of the electromagnetic flux is shown bythe dotted lines 56 in Fig. 7. Since the sum of these gaps remainsconstant when the armature is displaced, the permeance of the flux pathis constant and the flux is therefore independent of the displacement ofthe armature.

The flux from the permanent magnet passes in both directions through thecasing 26. One branch'of the flux crossing gap [6 travels back throughthe upper portion Fig. 1 (left-hand portion Fig. 7) of the armature tothe permanent magnet. The other branch of the flux passes through gap 54and the lower part Fig. 1 (righthand portion Fig. '7) of the armatureback to the electromagnet as shown by the full lines 51 in Fig. '7. Theflux 51 from the permanent magnet thus aids the electromagnetic flux inone gap, gap 54, as shown in Fig. 7 and opposes the electromagnetic fiuxin the other gap, l6, asshown in Fig. '7. Since the permanent magneticflux increases onone side and decreases onthe other side when thearmature is displaced, the total flux through the permanent magnet tendsto remain constant. The permeance of the permanent magnet being lowcompared with the remainder of the permanent magnet flux path'includingthe gap, renders slight any tendency of an increase of the gap in oneflux path and a corresponding decrease of thegap in the parallel fluxpath to vary the total flux and thus accentutes the tendency of thetotal flux of the permanent magnet to remain constant.

It can be shown mathematically that the force exerted by the armaturevaries linearly with the electromagetic flux andwith the gap at one endof the armature.

If we denote the total flux through the permanent magnet @0," and assumeit to be constant, and call the permanent magnet flux in gap I6 @m" andthat in gap 54 em, then The force on the armature is the differencebetween the forces at the two ends, proportional to the squares of thefluxes. Assuming an electromagnetic flux as shown in Fig. 7 and denotingthe electromagnetic flux @r,"

From the above equation it will be noted that as the permanent magneticflux is constant and the total gap G is constant, the force exerted onthe armature varies as a straight line function of the electromagneticflux Iu and the gap 9 at one end ,of the armature. This may berepresented by a series of parallel lines for the different currents asshown in Fig. 8, thus the constant current curves which also represent aconstant electromagnetic flux are parallel equally spaced straightlines. Now drawing the spring characteristic curve as shown by thedotted line in Fig. 8 on top of the constant current curve to obtain theequilibrium points, it will be noted that the equilibrium points areequally spaced. Reversing the current and electromagnetic polarity onlyreverses the direction of displacement.

I have thus provided a magnetic device in which the displacement of thearmature is proportional to the current impressed on the electromagnets.Although the coils may be connected in series, in order to provide asafety factor the two solenoids or electromagetic coils ar connected inparallel so that the burning out of either coil will not render thedevice inoperative. As shown in Fig. 1, the current supplied to theelectromagnets may be regulated by means of a potentiometer having acontactor 58. Movement of the contactor 58 to one side of the midpointof the potentiometer will direct current in one direction through coilsI II and I4 and movement of contactor 58 to the other side of themidpoint will direct current in the opposite direction to those coils.

This device shown in Fig. 1 may be used in any of various types ofcontrol. It is particularly useful in controlling the pitch of apropeller to control the speed of an engine driving a controllable pitchpropeller. By actuating the contactor 58 by. a speed responsive devicethe propeller pitch may be varied to maintain constant engine speed orthe device may be used to maintain the speed of an engine in accordancewith that of a preselected standard speed in the manner shown in Fig. 4of Patent 2,258,462 in which one of the alternators may be driven eitherby an engine with which it is desired to compare the speed of thecontrolled engine or it may be driven by a reference device such as aconstant speed motor with which it is desired to match the speed of thecontrolled engine.

The electrical signal imparted to the electromagnet coils can beimparted by any other suitable device such as an electronic controlwhich will impress a voltage on the coils proportional to the variationin speed from a reference speed.

The cone shaped gap I6 (or 54) provides a decreased length of gap and anincreased surface area over that which would be provided by a fiat endedarmature and thus decreases the reluctance of the flux path between thearmature and the end plate. The above described magnetic device providesan armature whose movements are proportional to the impressed voltage orcurrent in the electromagnet and whose movement is linear through thecenter of the device. By such a construction a direct connection betweenthe armature and the linearly movable control valve is attained so thateach movement of the armature results in a corresponding and equalmovement of the control valve with no opportunity for lost motion. a

While the results shown in Fig. 9 might be theoretically possible underideal conditions, leakage effects and variations in the permanent magnetflux as the armature is displaced will cause slight departures from aperfect straight line. In the practical results obtained, it has beenfound, however, that magnetic devices which have been made will producecurves similar to that shown in Fig. 10 by means of which an entirelysatisfactory governor control for propeller pitch can be obtained. Itshould be noted that in the curve in Fig. 10 the displacement is alongastraight line substantially proportional to the current for aconsiderable displacement from the zero position. The deflection whichoccurs in'this curve at the higher currents and shows a largerproportional displacement for the increase in current at the highcurrents occurs so far out on the curve as to not interfere with theoperation in practice.

What I claim as new and desire to secure by Letters Patent is:

1. A magnetic device comprising a cylindrical casing having end platesand enclosing an an nular permanent magnet, an annular electromagneticcoil or solenoid, and an axially movable armature all arrangedsubstantially co-axial with said casing, means urging said rmature intoan intermediate position between said end plates. said armature, casingand end plates completing a circuit of magnetic material, except forgaps between opposite ends of said armature and said end plates, for themagnetic flux of said electroao'ia'las magnet, said permanent magnethaving one of its poles adjacent said armature and the other adjacentsaid casing and extending between intermediate portions of said casinand armature, said casing and armature/completing two magnetic flux flowpaths for the permanent magnet flux from an intermediate portion of saidarmature in opposite directions through said armature, said gaps andsaid casing.

2. A magnetic device comprising a hollow cylindrical casing, having anaxis, end plates on said casing, a pair of concentrically arrangedaxially spaced annular electromagnetic coils supported coaxial with, andwithin, said casing and having their magnetic axes substantiallyparallel to said axis, an annular permanent magnet arranged concentricwith, and between, said electromagnetic coils, and having magnetic axessubstantially normal to said casing axis, an axially movable armaturelocated within, and substantially eo-axial with, said annuli, andbetween said end plates, means for urging said armature into anintermediate position between said end plates.

3. A magnetic device comprising a hollow cylindrical casing having anaxis, end plates on said casing, an annular electromagnetic coilsupported co-axial with, and within, said casing and having its magneticaxis substantially parallel to said casing axis, an annular permanentmagnet arranged concentric with said electromagnetic coil and havingmagnetic axes substantially normal to said casing axis, an axiallymovable armature located within, and substantially co-axial with, saidannuli, and between said end plates, means for centering said armaturebetween said end plates, means outside of said casing for guiding saidarmature along the casing axis.

4. A magnetic device comprising a hollow cylindrical casing, end plateson said casing, a pair of concentrically arranged axially spaced annularelectromagnetic coils supported co-axial with, and within, said casing,an annular permanent mag' net arranged concentric with, and between,said electromagnetic cells, an axially movable armature located within,and co-axial with, said annull, means for centering said armaturebetween said end plates, said electromagnets being connected in parallelso as to cooperate to produce a common magnetic field, said armature,casing and end plates completing a circuit of magnetic material, exceptfor gaps between opposite ends of said armature and said end plates, forthe magnetic flux of said electromagnets, said permanent magnet havingone of its poles adjacent said armature and the other adjacent saidcasing and extending between intermediate portions of said casing andarmature, said casin and armature completing two magnetic flux flowpaths for the permanent magnet flux from an intermediate portion of saidarmature in opposite directions through said armature, said gaps andsaid casing.

5. A magnetic device comprisin a hollow cylindrical casing, end plateson'said casing, an'annular electromagnetic coil whose magnetic axisextends along its coil axis supported co-axial with, and within, saidcasing, an annular permanent magnet located between said end plates andhaving one pole adjacent its inner periphery and the other pole adjacentits outer periphery and arranged concentric with said electromagneticcoil, an axially movable armature located within, and co-axial with,said annuli, and between said end plates, means includin a pair ofopposed springs for centering said armature between said end plates,said centering means having a rate of increase of force with armaturedisplacement greater than the rate of increase of pull of the permanentmagnet with armature displacement.

6. A magnetic device comprising a hollow cylindrical casing, end plateson said casing, an annular electromagnetic coil supported co-axial with,and within, said casing, an annular permanent magnet arranged concentricwith said electromagnetic coil, an axially movable armature locatedwithin, and co-axial with, said annuli and between said end plates,means for centerin said armature between said end plates, said armatureand casing and end plates completing a circuit of magnetic material,except for gaps between opposite ends of said armature and said endplates, for the magnetic fiux of said electromagnets, said permanentmagnet having one of its poles adjacent said armature and the otheradjacent said casing and extending between intermediate portions of saidcasing and armature, said casing and armature completing two magneticflux flow paths for the permanent magnet fiux from an intermediateportion of said armature in opposite directions through said armature,said gaps and said casing, said centering means having a rate ofincrease of force with armature displacement greater than the rate ofincrease of pull of the permanent magnet with armature displacement.

7. A magnetic device comprisin an annular electromagnetic coil, anannular permanent magnet arranged concentric with said electromagneticcoil, a metal shell including end plates, arranged concentric with, andsurrounding said magnet and coil, an armature linearly movable through,and along the axis of, said magnet and coil, means at each end of saidarmature for decreasing the reluctance of the fiux path between saidarmature and end plates, said shell and armature acting as a flux pathfor both the permanent and electromagnets, said permanent magnet havingone of its poles adjacent said armature and the other adjacent saidshell and extending between intermediate portions of said shell andarmature, said shell and armature completing two magnetic flux flowpaths for the permanent magnet flux from an intermediate portion of saidarmature in opposite directions through said armature, said gaps andsaid shell.

8. A magnetic device comprising a casing, end plates on said casing, amovable armature guided for longitudinal movement within said casing andhaving its ends adjacent said end plates, an electromagnetic coil withinsaid casing and surrounding said armature, a permanent magnet withinsaid casing around said armature and located between the ends of saidcasing, between the ends of said coil, and between the ends of saidarmature, said permanent magnet havin one of its poles adjacent saidcasing and the other pole adjacent said armature.

9. Means for creating magnetic flux and a flux circuit comprising acasing, end plates on said casing, an armature within said casing andhaving its ends adjacent said end plates, an electromagnetic coil withinsaid casing and surrounding said armature, a permanent magnet fixedwithin said casing, around said armature and located between the ends ofsaid'casing, said coil and said armature, said permanent magnet havingone of its poles adjacent said casing and the other pole adjacent saidarmature, said casing, end plates and armature providin two separatemain flux paths for the permanent magnet flux from the permanent magnet,one path from the permanent magnet including the casing, armature andcasing, one end plate, the armature, and the other end plate back to thecasing,

10. .A control mechanism comprising a casing having end plates, acontrol armature arranged for longitudinal controlling movement betweenthe end plates and, with the casing, completing a circuit of magneticmaterial except for the gaps between opposite ends of said armature andsaid end pieces, a permanent magnet arranged with one of its polesadjacent said armature and located between and connecting intermediateportions of said casing and armature and, with the casing and armature,completing two separate magnetic flux circuits of magnetic materialexcept for the gap, each circuit including a portion of the armature,the gap between the end of the armature and the respective end piece,the end piece, a portion of the casing and the permanent magnet, anelectromagnetic coil arranged to produce magnetic flux in said firstmentioned circuit and resilient means for centering said armature.

11, A magnetic device comprising a casing, having end plates, anarmature spaced from and movable between said end plates to inverselyvary the spacing at each end but maintain the total spacing constant,resilient means having a substantially uniform spring rate centeringsaid armature between said end plates, means creating a steadynon-reversing magnetic flux and directing said flux into two fiow pathsthrough said casing, armature, and end plates, each path including thespace at a respective end of said armature, a portion of said flux flowpaths utilizing a common path, an electro magnet for creating a fluxflow through a single path including said armature, said casing and saidend plates, variable in quantity and direction for moving said armaturein accordance with the extent and direction of said electromagneticflux, said common path having a lower permeance for said electromagneticflux than said casing.

12. A magnetic device comprising an armature of magnetic material, and ayoke of magnetic material overlapping the ends of the armature, andestablishing two gaps, one at each end of the armature, between saidarmature and said yoke, said armature, yoke and both gaps. defining aclosed flux path, said armature being longitudinally movable toward saidyoke to increase one gap and equally decrease the other gap, meanshaving a substantially uniform rate of change of force for each unit ofdisplacement centering said armature within said yoke, means creating asteady flux flow in two paths, said paths extending from an intermediateportion of said armature in opposite directions through said armature,said gaps and said yoke to an intermediate portion of said yoke, andback through the flux creating means to said intermediate portion ofsaid armature, and means for creating an independent variable flux flowthrough said closed path.

13. A magnetic device as claimed in claim 12 in which varying the fluxflow through said closed path will move said armature from said centralposition a distance linearly proportional to said variable flux flow, incombination with a control valve havin an axially movable valve elementand having a valve opening linearly proportional to the movement of saidelement from a central position, and means connecting said armature andsaid valve element to render said valve element axially movable by saidarmature a distance linearly proportional to the movement 01' saidarmature.

14. A device as claimed in claim 12 in which the means for creating theindependent variable flux flow is an electromagnet and in. which thearmature is movable in opposite directions from a centered position byreversal of current in said electromagnet, the variable flux assistingthe steady flux through one gap and opposing the steady flux through theother gap to move said armature in one direction and upon said reversalof current, opposing the steady flux through said one gap and assistingthe steady flux through the other gap to move said armature in theopposite direction.-

15. An electrically operated control valve movable from a centralposition a distance linearly proportional to the electrical inputtheretocomprising a cylindrical casing having end plates and enclosing anannular permanent magnet having one pole adjacent its inner peripheryand the other adjacent its outer periphery, an annular electromagneticcoil having an axially extending magnetic axis, and an axially movablearmature extending between said end plates and movable axially toincrease the gap between the armature and one end plate and equallydecrease the gap between the armature and the other plate, all arrangedsubstantially coaxial with said casing, said permanent magnet extendingbetween an intermediate portion of said armature and an intermediateportion of said casing, means having a uniform spring rate urging saidarmature into a central position, whereby selectively electricallyenergizin said electromagnet will move said armature from said centralposition a distance linearly proportional to the electrical input, anaxially movable valve element, having a valve opening linearlyproportional to its movement from a central position, arrangedsubstantially coaxial with said armature and means connecting saidarmature and valve element to render said valve element axially movableby said armature a distance linearly proportional to the movement ofsaid armature,

16. A valve as claimed in claim 15 in which said valve element and saidarmature are mov able in opposite directions from said central positionand are moved in one direction. from said central position by flow ofcurrent in one direction in said electromagnetic coil and are movable inthe opposite direction from said central position by reversal oi.current in said electromagnetic coil, the magnetic flux of said coilassisting the magnetic flux of said magnet through said armatureadjacent one end plate and opposing the magnetic flux of said magnetthrough said armature adjacent the other end plate to move said armaturein one direction and upon said reversal of current, opposing the flux ofsaid magnet adjacent one end and assisting the flux 01' said magnetadjacent said other end to move said armature in the opposite direction.

17. A magnetic device comprising an armature of magnetic material, and ayoke of magnetic material overlapping the ends of the armature andestablishing two gaps, one at each end 01 the armature, between saidarmature and said yoke, said armature, yoke and both gaps, defining aclosed flux path, said armature being longitudinally movable toward saidyoke to increase one gap and equally decrease the other gap, meanshaving a substantially uniform rate of change of force for each unit ofdisplacement centering said armature with said yoke, a permanent magnet,located between said armature and said yoke and intermediate the ends ofsaid armature with one pole adjacent said armature and the otheradjacent said yoke, said armature and yoke providing two magneticfluxdlovy paths for said permanent magnet flux froifan intermediateportion of said armature in opposite directions through said armature,said gaps and said yoke, and means for creating an independent variableflux flow through said closed path.

STANLEY G. BEST.

' REFERENCES CITED The following references are of record in the file ofthis patent:

Number Number 15 398,331 1 ,184

12 UNITED STATES PATENTS Name Date Haines Jul 19, 1898 Simon Oct. 24,1911 Bliss May 7, 1912 White July 2, 1918 Campbell June 21, 192i LedererJuly 5, 1927 West May 23, 1939 Lakates July 25, 1939 Kennedy -1 July 1,1947 FOREIGN PATENTS Country Date France of 1909 Great Britain of 1883

